qemu: blockCopy: Allow reuse of raw image for shallow block copy

The documentation states that for shallow block copy the image has to
have the same guest visible content as backing file of the current
image if the file is being reused. This condition can be achieved also
with a raw file (or a qcow without a backing file) so remove the
condition that would disallow it.

(This patch additionally fixes crash described in
 https://bugzilla.redhat.com/show_bug.cgi?id=1215569 )

src/qemu/qemu_driver.c

@@ -17122,11 +17122,17 @@ qemuDomainBlockCopyCommon(virDomainObjPtr vm,
     if (qemuDomainDetermineDiskChain(driver, vm, disk, false, true) < 0)
         goto endjob;
 
+    /* clear the _SHALLOW flag if there is only one layer */
+    if (!disk->src->backingStore)
+        flags &= ~VIR_DOMAIN_BLOCK_COPY_SHALLOW;
+
+    /* unless the user provides a pre-created file, shallow copy into a raw
+     * file is not possible */
     if ((flags & VIR_DOMAIN_BLOCK_COPY_SHALLOW) &&
-        mirror->format == VIR_STORAGE_FILE_RAW &&
-        disk->src->backingStore->path) {
+        !(flags & VIR_DOMAIN_BLOCK_COPY_REUSE_EXT) &&
+        mirror->format == VIR_STORAGE_FILE_RAW) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
-                       _("disk '%s' has backing file, so raw shallow copy "
+                       _("shallow copy of disk '%s' into a raw file "
                          "is not possible"),
                        disk->dst);
         goto endjob;